Ultra-high current density thin-film Si diode

ABSTRACT

A combination of a thin-film μc-Si and a-Si:H containing diode structure characterized by an ultra-high current density that exceeds 1000 A/cm 2 , comprising: a substrate; a bottom metal layer disposed on the substrate; an n-layer of μc-Si deposited the bottom metal layer; an i-layer of μc-Si deposited on the n-layer; a buffer layer of a-Si:H deposited on the i-layer, a p-layer of μc-Si deposited on the buffer layer; and a top metal layer deposited on the p-layer.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC3699GO10337 between the United States Department of Energy andthe National Renewable Energy Laboratory, a division of the MidwestResearch Institute.

TECHNICAL FIELD

The invention relates to ultra-high current density thin-film diodes anda process of making the same utilizing a hot-wire chemical vapordeposition (CVD) technique at low substrate temperatures. The currentdensity produced exceeds 1000 A/cm², which is a record for thin filmdiodes. The diode is characterized by a good n-factor of about 1.8 andexcellent rectification of over 10⁷ at ±1.5V. The ultra-high currentdensity thin-film diode may be utilized in devices such as small areamemory, imaging detectors, high-density displays, and other low cost andflexible substrate electronic applications such as plastics, as well asdevices on paper.

The development of this ultra-high current density thin-film Si diode issimple and inexpensive and can be scaled up. It represents a significantadvance in replacement of complicated and expensive thin filmtransistors that are currently pervasive in all thin film consumerdevices, in that it directly addresses and significantly increases thedensity of the elements or pixels.

BACKGROUND ART

Thin film diodes are in widespread use, and in general, thin filmmanufacturing techniques are less expensive and produce higher yieldsthan wafer scale processing techniques used to fabricate crystalline or“discrete” diodes. Nevertheless, known thin film diodes havecharacteristics which are poorly suited for many applications.

For example, the rigid substrates on which thin film diodes arefabricated prohibits their use in applications in which the device mustbe physically deformed. Further, contaminants from metallic contactlayers frequently react with the semiconductor body during processing,and thereby degrades the diode's electrical characteristics. Since thediodes are typically used with other semiconductor devices, the diodesmust be separately fabricated and interconnected.

U.S. Pat. No. 5,155,565 discloses an amorphous silicon thin film p-i-nsolar cell and Schottky barrier diode on a common substrate, comprising:

a substrate;

a first conductive layer on the substrate;

an unseparated amorphous silicon ohmic contact layer over a solar cellportion and a diode portion on the first conductive layer;

one or more layers of amorphous silicon forming a diode body over thediode portion on the ohmic contact layer, including a layer of n-typesilicon doped to a concentration of 10¹⁸ to 10²⁰ atoms per cubiccentimeter with an element from Group V on the periodic table;

at least two layers of amorphous silicon forming a p-i-n solar cell bodyin conjunction with the ohmic contact layer over the solar cell portionon the ohmic contact layer, adjacent to and spaced from the diode bodyto form a separation between the solar cell body and the diode body;

insulating material within the separation between the diode body andsolar cell body, the diode body and solar cell body electricallyinterconnected by the first conductive and ohmic contact layers; and

a second conductive layer on the diode body and on the solar cell body,the diode body forming a Schottky barrier with the second conductivelayer.

A high density, optically corrected, micro-channel cooled, V-groovemonolithic laser diode array is disclosed in U.S. Pat. No. 5,828,683.The laser diode array comprises:

a substrate having an upper surface and a lower surface;

a plurality of v-grooves formed in the upper surface; a metalizationlayer formed on the upper surface and within the plurality of v-grooves;

a metalization break formed in each v-groove of the plurality ofv-grooves; and

a plurality of laser diode bars, wherein a single laser diode bar of theplurality of laser diode bars is placed within each v-groove of theplurality of v-grooves.

U.S. Pat. No. 6,229,153 B1 discloses a high peek current densityresonant tunneling diode comprising:

a) a substrate of nominally exact (100)+/−0.5° GaAs;

b) a multilayer resonant tunneling diode structure grown on the (100)GaAs substrate, the resonant tunneling diode structure comprising aquantum well layer of low band-gap material between barrier layers ofAlGaAs, and wherein the material of the quantum well layer is selectedsuch that the second energy level of the quantum well layer is at orslightly above the conduction band edge in GaAs, the quantum well layergrown to be a strained layer with smooth interfaces with the barrierlayers.

R. A. Gibson et al., in RECENT DEVELOPMENTS IN AMORPOHOUS SILICON p-njunction devices, Journal Of Non-Crystalline Solids, 35 & 36 (1900)725-730 North-Holland Publishing Company, disclose amorphous Si p-njunctions with various doping profiles prepared by the glow dischargeprocess to investigate the effect of the barrier profile on theelectrical properties of the diodes. The highest current densities, upto 40 A/cm², is obtained with n⁺-i-p⁺ structures. Under AM-1illumination, photovoltaic p⁺-i-n³⁰ cells generate open circuit voltagesof 0.7V and short-circuit currents up to 10 mA/cm², corresponding toefficiencies between 3 and 4%. Diode quality factors are alsoinvestigated.

There is a need for a thin film diode that tolerates a high forwardcurrent density and is capable of many potential applications inconsumer electronics, such as memory devices, photo-imaging detectors,and flat panel displays. The development of a simple and inexpensiveultra-high-current density thin-film Si diode would have a great impactfor replacing the complicated and expensive thin film transistors thatcurrently dominate all thin film consumer devices, and for significantlyincreasing the density of the elements or pixels in these consumerdevices.

DISCLOSURE OF THE INVENTION

One object of the present invention is to provide a thin film diode thattolerates a high forward current density.

Another object of the present invention is to provide a thin film Sibased diode with a forward current density of over 1000 A/cm², that ischaracterized by very good rectification of over 7 orders of magnitudeat +/−1.5V.

A further object of the present invention is to provide a thin film Sibased diode with a forward current density of over 1000 A/cm² whereinthe diode has the simple structure of substrate/metal/n/i/b/p/metal,wherein the n—factor that quantifies the diode is about 1.8 and theturn-on voltage is less than 1V.

The thin film Si based diode is fabricated using a hot-wire chemicalvapor deposition (HWCVD) technique. The diode formed has a simplestructure of: substrate/metal/n/i/b/p metal. The process temperaturerange for fabricating this diode is from about 140° C. to about 160° C.for all layers, and this range is much lower than that of existing thinfilm diode processes. These lower processing temperatures enable thediode to be fabricated on a low cost substrate, such as plastic. Duringprocessing, a thin interface buffer layer is inserted between the i andp layers, and an Al top contact layer is formed using an e-beam orthermal deposition. The area of the diode formed is less than 1 mm².Further, the diode may also be fabricated using other structures, suchas Schottky and p-i-n and plasma enhanced CVD techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the vacuum chamber configuration in which the ultra highcurrent density thin film Si diode is produced using hot wire chemicalvapor deposition (HWCVD).

FIG. 2 is a graph showing current density (A/cm²) versus voltage in asemi-log plot for a thin-film Si based diode with a forward currentdensity of over 1000 A/cm².

FIG. 3 is a graph depicting current density versus voltage in a semi-logplot for five different diode structures such as Schottky, combinationof n/i of a-Si/a-Si, a-Si/μc-Si, mild μc-Si/μc-Si middle, and highμc-Si/μc-Si.

FIG. 4 is a graph depicting current density for structures comprisingn/i layers of a-Si/a-Si, a-Si/μc-Si, and high μc-Si/μc-Si based diodesat −1V and +1V.

FIG. 5 is a graph depicting J (A/cm²) versus voltage in a semi-log plotfor various area ultra high density thin film μc-Si based diodes of theinvention that were characterized by forward current densities, where Jis repeatedly greater than 1000 A/cm². The size of the diodes isindicated in the figure.

FIG. 6 is a graph showing current versus voltage in linear plot for apolymer substrate diode of the invention wherein the diode wassuccessfully processed at about 140° C. for the polymer substrate.

FIG. 7 is a graph depicting current versus voltage in a semi-log plotfor a polymer substrate diode in which the n-factor quantified the diodeat about n=1.6 and wherein the process temperature proceeded at about140° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

Reference is now made to FIG. 1 which shows a vacuum chamber 10 in whicha ultra high current density thin film Si diode is prepared by hot wirechemical vapor deposition. Basically, the vacuum chamber comprises aheater 11 for the substrate 12. A filament 13, preferably W, is heatedto a temperature of about 2000° C. by passing an AC current at about 16amps there through and this filament temperature is used for all of thehot wire layers. The filament for the spiral section of the W wire ispreferably of a dimension of about 0.5 mm in diameter, the wire iscoiled or spiraled in a 6 mm diameter, and the filament is about 6 cmlong. The distance between the substrate and the W filament will rangefrom about 4 to about 5 cm as depicted by the arrow 14. In general, theHWCVD procedure for preparing the ultra high current density thin filmSi diode entails loading the metal coated substrate which has beendeposited by e-beam into the HWCVD chamber 10; heating the substrate 12to a temperature of between about 140° C. to about 160° C. and creatinga vacuum utilizing a throttle valve 15 of the vacuum chamber to obtain avacuum below about 10⁻⁶ torr. Thereafter, the filament is turned on bypassing an AC current of about 16 amps there through to provide afilament temperature of about 2000° C., and opening a gas inlet 16having gas valves associated therewith, so that each gas valve is fixedat a pre-set flow rate for each layer as set forth in Table I below,which gives the HWCVD parameters for the high density device.

TABLE I 5% PH₃ 3.1% TMB Dep. Deposition Thickness SiH₄ H₂ in H₂ in HeTime Pressure Step technique (Å) (sccm) (sccm) (sccm) (sccm) (mn.)(mTorr) 1 substrate Glass 2 Bottom Cr e-beam 1000 metal 3 n-layer μc-SiHWCVD 200 3 45 3 0 1 22 4 i-layer μc-Si HWCVD 1500 10 36 0 0 3 20 5Buffer- a- HWCVD 350 10 6 0 0 0.66 10 layer Si:H 6 p-layer μc-Si HWCVD400 3 27 0 6 2 22 7 Top Al, e-beam 700 metal

Note: TMB is trimethylboron. A 160° C. substrate temperature and a 2000°C. W filament are used for all HW layers. A 0.5 mm in diameter spiraltungsten wire coiled in 6 mm in diameter and 6 cm long is used as thefilament. The deposition procedure is: load the metal coated substrateinto the HWCVD chamber; heat the substrate to 160° C. and pump down thevacuum to below 10⁻⁶ torr; turn on the filament by passing an AC currentto 16 A (this gives about a 2000° C. filament temperature) and openingeach gas valve to the pre-setting flow rate listed in the above tablefor each layer. Each layer's deposition is sequential from step 3 tostep 6 (see the table) with less than a 1 minute break between thelayers. The total process time is less than 10 minutes. The depositionsare layered sequentially as is shown in steps 1-7 of Table I. The μcrepresents microcrystals, as opposed to amorphous silicon,nano-crystalline silicon or polycrystalline silicon.

In FIG. 2, a current density-voltage characteristic is shown for athin-film Si-based diode with a forward current density of over 1000A/cm². This number is 100 times better than the best published value todate. This diode not only has a high forward current density but also avery good rectification of over 7 orders of magnitude at +/−1.5V. Also,the n-factor that quantified the diode is about 1.8 and the turn-onvoltage is less than 1V.

To achieve this ultra high-current density diode, it has been foundthat:

1. μc Si thin film based n-i-p diodes give a better current density andlower turn-on voltage than an a-Si:H based thin film diode;

2. The serial resistance including the probe contact at the front andback electrodes plays a key role in improving the current density;therefore, the smaller areas of diode with low current that pass throughthe diode and reduce serial resistance effect are key steps to improvethe current density;

3. Since a-Si:H based diodes have a low reverse leakage current, it isclear that the high-rectified diode comes from the combination of ana-Si:H and μc-Si material;

4. The i-layer thickness is other key parameter to give high currentdensity. In the end, space charge limit current (SCLC) will be the upperlimit for the current. The thinner i-layer will increase the SCLCalthough the current of the diode is primarily limited by the serialresistance; and

5. Slightly P doped i-layer gives a high current. The i-layer in thediode is unintentionally light P doped from the contaminated chamberafter the n-layer growth.

The J-V characteristics of the high current density diode of FIG. 2 iswith the dimensions of 100 μm×100 μm. A size of a 10 μm×10 μm diode hasbeen made to achieve 1000 Å/cm². Currently, the size is limited by theprocess.

The current density versus voltage graph for FIG. 3 represents fivedifferent diode structures, wherein the basic diode is characterized by:SS/n-i-p/Pd with an area of 0.025 cm².

In the context of the invention, it can be seen from FIG. 4 thatcurrents are provided for structures comprising n/i of a-Si/a-Si,a-Si/μc-Si, and highly μc-Si/μc-Si based diode at −1V and +1V. It isclear from FIG. 4 that all μc-Si diode structures give the highestcurrent.

FIG. 5 provides a graph wherein diodes of certain specifications setforth therein provide repeatable high J>1000 A/cm² diodes.

The graph of FIG. 6 demonstrates that the invention process may besuccessfully carried out at a low temperature of about 140° C. Thecurrent versus voltage graph in linear plot is for a polymer substratediode of the invention process conducted at 140° C.

The graph showing current versus voltage in semi-log plot for a polymersubstrate diode in FIG. 7 is one wherein the process temperature iscarried out at 140° C. The n-factor quantified for the diode is at aboutn=1.6.

In the preferred embodiment of the invention, the ultra-high currentdensity, thin-film Si based diode structure will be composed of thelayers shown in Table II.

TABLE 2 Structure Materials Metal Al, Au, Pd. p-layer μc-Si 400 Åb-layer a-Si:H 350 Å i-layer μc-Si 1500 Å n-layer μc-Si 200 Å metal Cr,Ti, Pd, etc. substrate SS, glass, or polymer

It is to be understood that the present invention is not limited to theembodiments disclosed herein, which are exemplary only, and encompassesall such forms thereof that come within the scope of the claimshereinafter set forth.

1. A thin film diode structure characterized by an ultra-high currentdensity that exceeds 1000 A/cm², said structure containing a combinationwherein a buffer layer of an a-Si:H is interfaced between an i-layer ofμc-Si and a p-layer of μc-Si, said thin film diode comprising: a) asubstrate; b) a bottom metal layer disposed on said substrate; c) ann-layer of μc-Si deposited on said bottom metal layer; d) an i-layer ofμc-Si deposited on said n layer; e) a buffer layer of a-Si:H depositedon said i layer; f) a p-layer of μc-Si deposited on said buffer layer;and g) a top metal layer deposited on said p layer.
 2. The thin filmdiode structure of claim 1, wherein said substrate is selected from thegroup consisting of stainless steel, quartz, glass, a polymer, or paper.3. The thin film diode structure of claim 2, wherein said polymer is aplastic.
 4. The thin film diode structure of claim 3, wherein saidplastic is selected from the group consisting of a polyimide or apolyester.
 5. The thin film diode structure of claim 2, wherein saidsubstrate is stainless steel.
 6. The thin film diode structure of claim2, wherein said substrate is quartz.
 7. The thin film diode structure ofclaim 2, wherein said substrate is glass.
 8. The thin film diodestructure of claim 2, wherein said substrate is paper.
 9. The thin filmdiode structure of claim 2, wherein said substrate is stainless steeland wherein said bottom metal layer in b) is selected from the groupconsisting of Cr, Ti or Pd.
 10. The thin film diode structure of claim9, wherein said n-layer in c) is a μc-Si of a thickness of about 200 Å.11. The thin film diode structure of claim 10, wherein said i-layer ind) is a μc-Si of a thickness of about 1500 Å.
 12. The thin film diodestructure of claim 11, wherein said buffer layer in e) is an a-Si:H of athickness of about 350 Å.
 13. The thin film diode structure of claim 12,wherein said p-layer in f) is a μc-Si of a thickness of about 400 Å. 14.The thin film diode structure of claim 13, wherein said top metal layerin g) is selected from the group consisting of Al, Au or Pd.
 15. Amethod of producing a combination thin film μc-Si and a-Si:H containingdiode structure using hot wire chemical vapor deposition, and saidstructure being characterized by an ultra-high current density thatexceeds 1000 A/cm², comprising: a) placing a substrate selected from thegroup consisting of stainless steel, quartz, glass, a polymer or paperin a vacuum chamber; b) depositing a bottom metal layer on saidsubstrate disposed in a vacuum chamber using an e-beam; c) depositing ann-layer of μc-Si on said metal layer using hot wire chemical vapordeposition wider a vacuum of below about 10⁻⁶ torr; d) depositing ani-layer of μc-Si layer on the n-layer by hot wire chemical vapordeposition at a vacuum below about 10⁻⁶ torr; e) depositing a bufferlayer of a-Si:H on said i-layer by hot wire chemical vapor deposition ata vacuum pressure of below about 10⁻⁶ torr; f) depositing a p-layer ofμc-Si layer on said buffer layer by hot wire chemical vapor depositionat a vacuum pressure of below about 10⁻⁶ torr; and g) depositing a topmetal layer on said p-layer by e-beam in the absence of a vacuum. 16.The method of claim 15 wherein said substrate is selected from the groupconsisting of stainless steel, quartz, glass, a polymer or paper. 17.The method of claim 16 wherein said polymer is a plastic.
 18. The methodof claim 17 wherein said plastic is selected from the group consistingof a polyimide or a polyester.
 19. The method of claim 16, wherein saidsubstrate is stainless steel.
 20. The method of claim 16, wherein saidsubstrate is quartz.
 21. The method of claim 16, wherein said substrateis glass.
 22. The method of claim 16, wherein said substrate is paper.23. The method of claim 16 wherein said substrate is stainless. steeland wherein said bottom metal layer in step b) is selected from thegroup consisting of Cr, Ti or Pd.
 24. The method of claim 23 whereinsaid n-layer in step c) is a μc-Si of a thickness of about 200 Å. 25.The method of claim 24 wherein said i-layer in step d) is a μc-Si of athickness of about 1500 Å.
 26. The method of claim 25 wherein saidbuffer layer in step e) is an a-Si:H of a thickness of about 350 Å. 27.The method of claim 26, wherein said p-layer in step f) is a μc-Si of athickness of about 400 Å.
 28. The method of claim 27 wherein said topmetal layer in step g) is selected from the group consisting of Al, Auor Pd.